Electronic control units (ECUs) monitor and control the low-level systems of a vehicle. For example, the ECUs may control and/or monitor the lighting system, the engine, the power locks, the power windows, the power train, the HVAC system, and the battery management, etc. ECUs may have sensors (such as a knock sensor) that are connected to the ECU by a wire harness. Wires in the wire harness have impedance values (measured in ohms (Ω) and/or milliohms (mΩ)). Some sensors and/or ECUs are sensitive to drops in voltage. For example, an ECU that normally operates at an input voltage of 14 volts (V) may not operate properly if the input voltage is 13V or less. Additionally, some sensors/ECUs draw 3 to 10 amps (A) of current from the battery. According to Ohm's Law (voltage=current×resistance), if the ECU has a 1V tolerance and draws 10 A, the wire harness can have a maximum resistance of 100 mΩ. Additionally, in some vehicles that include a start-stop system, the vehicle system voltage may occasionally drop. In such cases, the maximum resistance of the wire harness determined to be even lower to take into account the occasional voltage drops.
Digital multimeters and ohmmeters measure resistance. However, the resolution and/or accuracy of typical commercial meters cannot make accurate milliohm measurements. For example, if a meter has a range of 600Ω, a resolution of 0.1Ω, and an accuracy of 0.5%+0.2Ω, the potential error of the meter is up to ±3.2Ω (600Ω*0.005+0.2Ω). In such an example, if the meter indicated the resistance of the wire harness was 200 mΩ, the actual resistance could be from 0 mΩ to 401 mΩ. Such a meter cannot measure resistance of the wire harness with enough accuracy.